Suggest Treatment For An Infected Blister On My Penis

i have a small blister looking thing on the shaft of penis. i remember cutting it shaving in the same spot a while back and it was a red and irritable but it went away with time. now there is a blister in the same spot and i attempted to pop it because it looked like an ingrown hair and nowit is open and willl not heal. do you have any idea what it may be? im afraid it could be an std. i recieved oral sex about two months ago for the first time ever from my girlfriend who i believe does not have any stds. i havent done anything sexual with anyone else before her.